Field of the Invention
The present invention relates to a phase front/phase tail synchronized modulation circuit, and in particular to a middle phase power-fetching type phase front/phase tail synchronized modulation circuit, its output can be modulated synchronously from the middle phase, such that its phase front and phase tail can be refracted inward or expanded outward at the same time, so that the phase front turned-on output voltage average value and the phase tail turned-on output voltage average value complement each other, in achieving a stable and modulated power supply.
The Prior Arts
In general, the phase modulation circuit can be classified into a phase front modulation circuit and a phase tail modulation circuit, and it is utilized extensively in our daily life. In particular, it is used mostly frequently in modulation of illuminance of a light and rotation speed of an electric fan motor.
The conventional phase tail modulations are realized mostly through using a TRIAC transistor switches. The drawback of this type of phase tail modulation is that, once it is triggered to turn-on, it can not be turned off until the end of the phase cycle. In addition, the instantaneous activation current is rather large, thus it is apt to create a large peak current and large Electromagnetic Interference (EMI), and is liable to cause magnetic or light filament vibration noise, to cause light blinking. On the other hand, conventional phase front modulations are realized mostly through an MOSFET transistor switch. In its control, it can first be turned on and then turned off, to trigger accurately the zero phase activation. In application, its advantage is that, it will not produce large peak current, so that in activation, it will not generate large EMI, to reduce filament noise of incandescent light, so that its modulation effect is much better than that of the conventional TRIAC phase tail modulation.
The phase modulation circuit of the Prior Art is realized through phase front modulation or phase tail modulation. In other words, in a half cycle of voltage waveform, only a single phase modulation of either phase front or phase tail is performed. However, when the phase of a power supply is drifting, the increase of width of phase front or phase tail will cause the light to become brighter, while the decrease of width of phase front or phase tail will cause the light to become dimmer, thus it is liable to cause blinking and noise. For this reason, an improvement is proposed in U.S. Pat. No. 7,099,132B2, wherein the circuit is turned on through combining the phase front and phase tail turn-ons, to effectively overcome the shortcomings of the either type of the two single phase modulations mentioned above.
However, quite a lot of the drawbacks still exist in U.S. Pat. No. 7,099,132B2. Refer to FIGS. 9 and 10 respectively for a circuit diagram of U.S. Pat. No. 7,099,132B2 according to the Prior Art; and a waveform diagram of phase front and phase tail modulation of U.S. Pat. No. 7,099,132B2 according to the Prior Art. As shown in FIGS. 9 and 10, in the positive half cycle of AC voltage, MOSFET Q01 is turned on first, then the positive voltage passes through diode FW1 to load LOAD, then it arrives at MOSFET Q01. When voltage of the positive half cycle reaches point PNC1, MOSFET Q02 is turned on, while MOSFET Q01 is turned off. When the voltage in the positive half cycle reaches point PC1, MOSFET Q02 is turned off, while MOSFET Q01 is turned on, till the end of the positive half cycle of voltage. In the negative half cycle of AC voltage, MOSFET Q11 is turned on first, then the negative voltage passes through diode FW2 to load LOAD, then it arrives at MOSFET Q11. When voltage of the negative half cycle reaches point PNC2, MOSFET Q12 is turned on, while MOSFET Q11 is turned off. When the voltage in the negative half cycle reaches point PC2, MOSFET Q12 is turned off, while MOSFET Q11 is turned on, till the end of the negative half cycle of voltage.
In the descriptions above, in the positive and negative half cycles, when MOSFETs Q01 and Q11 are turned on, the current will pass through diode FW1 or FW2, to create voltage drop of 0.7V to cause overheating. The shortcoming of this design is that, basically, when 1 A current is passed, it could cause power loss of 1.4 W. In case 2 A current is passed through, the heat thus produced could cause MOSFETs Q01 and Q11 to reach a temperature as high as 80˜90° C. Another shortcoming of this design is that, the two transistors Q02 and Q12 in the circuit are turned-on and turned-off by the divided voltage of AC power supply. The operation points of transistors Q02 and Q12 are adjusted respectively through using P01 and D11, thus they can not fulfill the demand of complete synchronization. Therefore, the imbalance of output power between the positive half cycle and the negative half cycle could cause blinking of lights. In particular, when the voltage is not stable and can not be synchronized, the blinking of lights is especially evident. A further shortcoming of that U.S. patent is that, it can not derive the power it requires from the AC voltage waveform, a separate power circuit have to be designed to provide power. In addition, that circuit is not capable of preventing overheating and overload. As such, even when overheating occurs, the circuit can not be turned off, such that it will continue to heat up, until the circuit is burned out.
Therefore, presently, the design and performance of phase front/phase tail modulation circuit is not quite satisfactory, and it has much room for improvement.